This invention relates generally to variable-level liquid rheostats of the type currently used, for example, for starting electric motors, the starting of which necessitates the insertion, in the corresponding excitation circuit, of a resistance which, initially high, is required to decrease progressively as starting proceeds.
The invention may equally well be applied to the starting of stator-starting motors, and to the starting of rotor-starting motors, and particularly to the starting of short-circuit rotor motors.
Generally speaking, the variable-level liquid rheostats used for this purpose comprise as described for example in the French Patent filed on June 27, 1977 under Pat. No. 77 18915 and published under Pat. No. 2,395,578, a tank, which contains electrolyte, a starting chamber, which is arranged above said tank and in the upper part of which are arranged electrodes, connecting terminals adapted to link said electrodes with the excitation circuit of the motor to be started, and a feed pipe fitted between said tank and said starting chamber for the introduction of electrolyte into the latter.
Initially, only a minimal quantity of electrolyte is present in the starting chamber, and therefore only the lower portion of the electrodes is immersed.
As a result, the latter then introduce a high resistance into the excitation circuit of the motor to be started.
A progressive invasion of the starting chamber by electrolyte is effected during starting, by appropriate actuating means, and for example by a pump, so that an increasing section of the electrodes is progressively immersed, and thus short-circuited.
Since only the emergent part of these electrodes continues to insert a resistance into the excitation circuit of the motor to be started, this resistance therefore decreases progressively, as required.
At the end of the starting thus effected, which implies stability of the speed of the relevant motor, and hence stability of the current flowing in its excitation circuit, the electrodes are disconnected, either by a contact controlled by a float, or by a contact time-delayed for this purpose.
In practice, a vent is usually provided at the upper part of the starting chamber, which places the upper part of said starting chamber in communication with the upper part of the tank, to prevent any compression of the atmosphere in the starting chamber with the progressive invasion of the latter by electrolyte, and which, forming a discharge sill, may at least occasionally function as an overflow aperture for this electrolyte.
Although such variable-level liquid rheostates have given and may still give satisfaction, their use, at least in certain applications, may give rise to difficulties.
In fact, they do not hitherto permit convenient control of the increase in speed of motors during starting, or in other words, their acceleration process, nor afterwards their setting to nominal speed.
Dealing with stator-starting motors, the reason is that, according to at least apparently conflicting demands: that the increase in speed must be made very rapidly, which requires the use of actuating means adapted to ensure a considerable flow of the electrolyte from the tank into the starting chamber, and that, conjointly, after the previous increase in speed, a constant current must be maintained during the end of starting, which requires a stabilisation of the level reached by the electrolyte in the starting chamber.
Dealing with rotor-starting motors, the reason is likewise that although, in a first phase, the increase in speed of the motor during starting must again be made rapidly, it is desirable that, in a second phase, the rise of the electrolyte into the starting chamber should be slow to permit the motor to complete its attainment of speed.
The present invention relates generally speaking to an arrangement permitting these diverse requirements to be satisfied in particularly simple and efficacious manner, and therefore to permit the increase in speed of a motor during starting to be controlled with high certainty.